Steering devices for the rear wheels of vehicles are known in which both a steering mechanism, in which the driving force of an electric motor is transmitted to the rear wheels of a vehicle in order to steer the rear wheels, and a controller, for controlling the operation of the electric motor in response to either the steering angle of the steering wheel or the wheels' speed, are provided. Such devices are disclosed, for example, in Japanese Unexamined Patent Application (hereinafter Kokai) No. Sho 57-44568, and Kokai No. Sho 61-46763. This type of rear wheel steering device, when compared to the type of device which steers the rear wheels by transmitting the steering power of the front wheels to the rear wheels by means of a rod, etc., has the advantages that even when the steering angle of the front wheels is zero, the rear wheels can be steered and the ease of control of the steering of the rear wheels is great.
To counteract motor failure in this type of prior art steering device for rear wheels, a clutch for interrupting the power from the electric motor to the rear wheels, and a centering spring which holds the rear wheels in a centered position are provided in the steering mechanism.
However, because the preset spring load of the centering spring is set at a fairly high value, there is the fear of instability because, when the motor fails during rotational operation, the rear wheels are suddenly returned to the centered position. Also there is the problem that, to counteract the high spring load, the electric motor is made large and with a large output. Also, the rear wheel steering is lacking in quickness.
In order to solve the above problems, it was thought to provide two electric motors in series so that when one motor fails, the other is used as a reserve. However, in this type of arrangement, because it is necessary to provide a clutch to cut off the power transmission line from the electric motor, the device must be made with large dimensions. Also, because the reserve drive motor is only rarely used, maintenance of its operation is also a problem.